1. Field of the Invention
This invention relates to an apparatus and a method for pumping a hot, erosive slurry of coal solids in a coal derived, water immiscible liquid. More particularly, the present invention relates to the use of a motive fluid which is miscible with the coal derived liquid of the slurry to produce a concentration gradient of coal solids through which the reciprocating action of a pump is transmitted.
2. Description of the Prior Art
In coal liquefaction processes, hot erosive slurries of coal solids in a coal derived, water immiscible liquid must be pumped. The temperature and erosive nature of the slurry is detrimental to the pumping apparatus by causing rapid wear and damage to the pump mechanism. Rapid wear and damage to the pump necessitate interruptions to the process and result in high maintenance costs.
Conventional apparatus and methods of pumping erosive slurries, such as ore slurries or sludgy water, have used a liquid medium which is immiscible with the slurry liquid and is acted upon by a reciprocating piston to pump the slurry. For example, U.S. Pat. No. 3,241,496 to Imani et al discloses a system for pumping erosive slurries which comprises a pumping apparatus with a remote check valve arrangement and an expanded chamber between the pump and check valve to prevent the abrasive particles in the slurry from entering the pump mechanism. The Imani et al apparatus employs a liquid medium which has a lower specific gravity than that of the slurry and is of such a nature that it does not mix with, dissolve into or react with the slurry to be pumped. A boundary layer is formed between the immiscible liquid medium and the slurry. A stabilizing arrangement is used in combination with the boundary layer formation in the chamber to prevent horizonal movement of the layer and, thus, inhibit intermixing of the slurry and the liquid medium.
The Imani et al system cannot be utilized to pump a hot, erosive slurry of coal solids and a coal derived, water immiscible liquid for use in a coal liquefaction process, since a liquid that is immiscible with the oily slurry liquid such as an aqueous medium, e.g., water, can have an adverse effect upon the system. Thus, it would not be possible to prevent mixing of the water and the coal slurry, and the water would become emulsified in the slurry liquid. The emulsification of the water and slurry liquid would cause erratic pump performance and excessive wear and damage to the pump mechanism due to the corrosive and non-lubricant nature of the aqueous fluid. Additionally, liquids such as water boil upon contact with the hot slurry which is normally at a temperature in the range of 250.degree. to 700.degree. F. (121.degree. to 371.degree. C.) thereby causing pump cavitation and foaming.